How to determine your Mini's differential ratio:
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1. Put your Mini in 4th gear. Except on some 5speed boxes, 4th gear is always 
1:1 ratio. With this situation, the only difference between your crankshaft and 
your wheels is the diff ratio. 

2. Make a mark on the crankshaft pulley. It's on the radiator side of your 
engine (or on the right side, standing in front of the engine bay if you have a 
post -97 Mini), the one most downwards. Don't use the pulley of the waterpump or 
the generator as they are not driven 1:1 from the crank. 

3. Make a mark on one of the wheels. 

4. Then push your Mini and count the revolutions of the crank pulley and the 
wheel (having some helping hand here helps a lot ;-} ) You should push it quite 
a while, counting only one revolution on the pulley is not exact. Better do 10 
or even more is you have the space. Of course you can measure the distance the 
car has travelled and calculate the turns of the wheels from the circumference.

5. Then divide the turns of the pulley by the turns of the wheel. For example, 
you have pushed the Mini until the pulley did 10 revs. The wheels did about 3 
turns, then your diff ratio is: 10/3 = 3,33 So it might be a 3,44:1 diff. 

It might have been 3,2 turns of the wheel, then the diff would rather be 3,1:1, 
so you see absolute exactness is necessary. So better push your Mini a few meter 
more.
 

 How to determine the speedo drive ratio:
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1. First you ned to know the differential ratio. (see above)
2. Disconnect the speedo cable from the speedometer. 
3. Mark the inner cable so you can count the turns of it. (Use a thin wire or 
   some tape, see picture here)
4. Make a mark on one of the wheels.
5. Put your Mini in neutral
6. Push it and count the turns of the speedo cable and the wheels. (procedure see 
   above)
7. Now you can calculate the speedo drive ratio using the following formula:

Rs = Nw / Ns * diff
   where 
Rs   = speedo drive ratio
Nw   = turns of wheels
Ns   = turns of speedo cable
diff = diff. ratio

Let's say you've counted 10 turns on the speedo cable and the wheels turned 8 
times, then Rs = 8 / 10 * 3,44 = 2,75 

It's the same as in the chapter above: the further you push it, the more 
exact is the calculation.

Now you need to know if the speedo drive suits your speedo. You can do this by 
using the little programm I've written (gearbox.exe) or see the next chapter.


 Finding the suiting speedo drive ratio for your speedometer and differential 
 ratio:
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1. First you'll have to determine how many turns per mile (or kilometer) your 
   speedometer wants to see. That's a little number written below the odometer. 
   On the later speedos which are mounted in front of the driver, this number is 
   written on the back of it, unfortunatelly. It'll say something like: 800 on a 
   km/h speedo or 1280 on a mph one.
2. Then you'll have to do a little bit of calculation:

In km/h:
Rs = ( 1000 * diff ) / (circum * number )

In mph:
Rs = ( 1610 * diff ) / (circum * number ) since 1 mile is 1.61 kilometers

   where 
Rs     = speedo drive ratio
circum = circumference of your tyre in m
         Example for 165/70/10 wheels
         circum = ( [165]*[70]/100*2 + [10]*25,4 ) * 3,14 = 1523,67mm = 1,524m )
number = the number you've read from the speedo
diff   = diff. ratio

Example for km/h:
diff = 3,44;circum = 1,524m; number = 800
Rs = (1000 * 3,44) / ( 1,524m * 800) = 2,822
Example for mph:
diff = 3,44; circum = 1,524m; number = 1288
Rs = (1610 * 3,44) / ( 1,524m * 1288) = 2,822

As you can see, in both examples your Mini will need the same speedo drive. 
That's logical as only the speedo has changed and the one in mph has the same 
gearing as the one in km/h, it just has a different labelling.